This application is the national phase under 35 U.S.C. xc2xa7 371 of PCT International Application No. PCT/JP00/09271 which has an International filing date of Dec. 26, 2000, which designated the United States of America and was not published in English.
The present invention relates to a multi-frequency array antenna that is used as a base station antenna in a mobile communication system, and is used in common for a plurality of frequency bands which are separated apart from each other.
Antennas such as base station antennas for implementing a mobile communication system are usually designed for respective frequencies to meet their specifications, and are installed individually on their sites. The base station antennas are mounted on rooftops, steel towers and the like to enable communications with mobile stations. Recently, it has been becoming increasingly difficult to secure the sites of base stations because of too many base stations, congestion of a plurality of communication systems, increasing scale of base stations, etc. Furthermore, since the steel towers for installing base station antennas are expensive, the number of base stations has to be reduced from the viewpoint of cost saving along with preventing spoiling the beauty.
The base station antennas for mobile communications employ diversity reception to improve communication quality. Although the space diversity is used most frequently as a diversity branch configuration, it requires at least two antennas separated apart by a predetermined spacing, thereby increasing the antenna installation space. As for the diversity branch to reduce the installation space, the polarization diversity is effective that utilizes the multiple propagation characteristics between different polarizations. This method becomes feasible by using an antenna for transmitting and receiving the vertically polarized waves in conjunction with an antenna for transmitting and receiving the horizontally polarized waves. In addition, utilizing both the vertically and horizontally polarized waves by a radar antenna can realize the polarimetry for identifying an object from a difference between radar cross-sectional areas caused by the polarization.
Thus, to make effective use of space, it is necessary for a single antenna to utilize a plurality of different frequencies, and in addition, the combined use of the polarized waves will further improve its function. FIG. 1 is a plan view showing a conventional two-frequency array antenna disclosed by Naohisa Goto and Kazukimi Kamiyama, xe2x80x9cDirectivity of Dual Frequency Co-Planar Array Antennaxe2x80x9d (Technical Report A.P81-40 of the Institute of Electronics, Information and Communication Engineers of Japan, Jun. 26, 1981). FIG. 2 is a partial view of the array antenna seen looking normally to the Axe2x80x94A line of FIG. 1. In FIGS. 1 and 2, the reference numeral 101 designates a ground conductor; 102 designates a dipole antenna that operates at a relatively low frequency f1; 103 designates a feeder for feeding the dipole antenna 102; 104 designates a dipole antenna that operates at a relatively high frequency f2; and 105 designates a feeder for feeding the dipole antenna 104. Thus arranging the dipole antenna 102 with a resonant frequency f1 and the dipole antenna 104 with a resonant frequency f2 on the same ground conductor 101 enables the two-frequency antennas to share the aperture. Here, although the description is made taking an example of the two-frequency array antenna for convenience sake, a multi-frequency array antenna, which is constructed by arranging three or more dipole antennas with different frequency characteristics on the same ground conductor, has an analogous configuration.
Next, the operation of the conventional antenna will be described.
The dipole antenna has a rather wideband characteristic with a band width of 10% or more. To achieve such a wide bandwidth, however, it is necessary for the height from the ground conductor to the dipole antenna to be set at about a quarter wavelength of radio waves or more. Besides, since the dipole antenna forms its beam by utilizing the reflection on the ground conductor, when the height to the dipole antenna is greater than the quarter wavelength, it has a radiation pattern whose gain is dropped at the front side. Therefore, it is preferable that the height from the ground conductor to the dipole antenna be set at about a quarter of the wavelength of the target radio waves. Furthermore, as the feeders 103 and 105 for feeding the dipole antennas, a twin-lead type feeder or coaxial line is usually used. Constructing the dipole antennas using a printed circuit board consisting of a dielectric board enables the twin-lead type feeder to be formed on the printed circuit board, offering an advantage of being able to obviate soldering and to facilitate its fabrication.
As for the foregoing array antenna comprising the dipole antennas 102 and 104 working at the frequencies f1 and f2, respectively, the two dipole antennas 102 and 104 are disposed at the heights different from the ground conductor 101: The dipole antenna 104 operating at the relatively high frequency f2 is placed closer to the ground conductor 101 than the dipole antenna 102 operating at the relatively low frequency f1. Furthermore, it is necessary for the array antenna to have such element spacing that can prevent grating lobes at respective operating frequencies. Since the element spacing of the dipole antenna 102 working at the frequency f1 differs from that of the dipole antenna 104 working at the frequency f2, their adjacent elements are disposed not to be overlaid on each other, to obtain the two-frequency characteristics.
With the foregoing configuration, the conventional array antenna has the following problems when it uses two frequencies. First, since the dipole antenna operating at the relatively low frequency f1 is greater in size than the dipole antenna operating at relatively high frequency f2, the former hinders the operation of the latter. In addition, radio waves which are radiated from the latter will induce excitation current in the former when they are coupled with the former, thereby causing reradiation. Thus, another problem arises in that the radiation directivity of the dipole antenna operating at the frequency f2 is disturbed by the effect of the dipole antenna operating at the frequency f1. Here, the disturbance of the radiation directivity of the dipole antenna operating at the frequency f2 appears periodically depending on the spacing between the dipole antennas operating at the frequency f1. The periodic disturbance causes the grating lobes in the array radiation directivity as illustrated in FIG. 3.
It is possible to reduce the disturbance of the radiation directivity of the dipole antenna operating at the frequency f2 caused by the reradiation, by disposing the dipole antenna operating at the frequency f2 over the dipole antenna operating at the frequency f1. In this case, however, since the height from the ground conductor becomes greater than a quarter of the wavelength of the radio waves of the operating frequency f2, there arises another problem in that the gain at the front of the antenna is reduced, and that null points, which are brought about by the reflection on the ground conductor in wide-angle directions, result in large distortion in the radiation directivity.
The present invention is implemented to solve the foregoing problems. Therefore an object of the present invention is to provide a multi-frequency array antenna that can reduce the degradation in the radiation directivity of the dipole antenna operating at the relatively high frequency when two frequencies share the aperture in common by weakening the effect of the dipole antenna operating at the relatively low frequency on the dipole antenna operating at the relatively high frequency.
According to one aspect of the present invention, there is provided a multi-frequency array antenna including a ground conductor with a flat surface or a curved surface, a plurality of linear antennas each mounted on the ground conductor to operate at an operating frequency, and feeders for feeding the plurality of linear antennas, the multi-frequency array antenna comprising: an array that is composed of the plurality of linear antennas by combining a plurality of linear antenna groups for respective operating frequencies to operate at least at two frequencies, each of the linear antenna groups including a plurality of systematically arranged linear antennas that operate at a particular operating frequency; and cranks formed on antenna elements constituting the linear antennas operating at the operating frequencies lower than a maximum frequency among the plurality of operating frequencies.
This offers an advantage of being able to reduce, when the multi-frequency array antenna operates at a frequency f2 higher than a frequency f1, the degradation in the radiation directivity of the linear antennas operating at the frequency f2 because the excitation current is reduced which is induced in the linear antennas operating at the frequency f1 by the inter-element As coupling, thereby suppressing the reradiation caused by the excitation current. In addition, this offers an advantage of being able to shrink the size of the linear antennas operating at the frequency f1 as compared with a conventional ordinary linear antenna operating at the frequency f1, because the former maintains the resonant length at the frequency f1 by the length including the cranks.
Here, the cranks formed in the linear antennas operating at a first operating frequency may have a height equal to a quarter of a wavelength of radio waves of a second frequency higher than the first frequency.
This offers an advantage of being able to sharply reduce, when the multi-frequency array antenna operates at the frequency f2, the degradation in the radiation directivity of the linear antennas operating at the relatively high frequency f2, because the excitation current is reduced which is induced in the linear antennas operating at the frequency f1 by the inter-element coupling with the linear antennas operating at the frequency f2, thereby suppressing the reradiation caused by the excitation current, and because each of the linear antennas operating at the frequency f1 can be seen as divided into a plurality of linear conductors with a length less than the resonant length because its crank start points and feeding point are assumed to be open at the operating frequency f2, and hence the excitation current caused by the inter-element coupling can be more efficiently reduced at the frequency f2.
The positions of the cranks on the antenna elements of the linear antennas operating at a relatively low frequency may be adjustable in accordance with positional relationships with the linear antennas operating at a relatively high frequency.
This offers an advantage of being able to sharply reduce, when operating the multi-frequency array antenna at the frequency f2, the degradation in the radiation directivity of the linear antennas operating at the relatively high frequency f2 because the excitation current is reduced which is induced in the linear antennas operating at the frequency f1 by the inter-element coupling, thereby suppressing the reradiation caused by the excitation current, and because the excitation current caused by the inter-element coupling can be efficiently suppressed because the excitation current is canceled out at positions at which the excitation current distribution takes the maximum value.
Each of the antenna elements constituting one of the linear antennas may comprise a plurality of cranks formed on each of the antenna elements.
This offers an advantage of being able to further reduce, when the multi-frequency array antenna operates at the frequency f2, the degradation in the radiation directivity of the linear antennas operating at the relatively high frequency f2 because the excitation current, which is induced in the linear antennas operating at the frequency f1 by the inter-element coupling, is canceled out at the positions of the cranks, thereby suppressing the reradiation caused by the excitation current.
Each of the plurality of cranks formed on each of the antenna elements, which constitute the first linear antenna operating at a first operating frequency, may have a length equal to a quarter wavelength of radio waves of any one of operating frequencies higher than the first operating frequency.
This offers an advantage of being able to markedly reduce the degradation in the radiation directivity of the linear antennas operating at the relatively high frequencies because the antenna elements can be seen as divided at the relatively high frequencies, and hence the excitation current caused by the inter-element coupling can be reduce at the relatively high operating frequencies by making the individual lengths of the subdivided linear conductors equal to or less than a quarter of the wavelength of the radio waves at the operating frequencies.
Each of the linear antennas with the cranks, which operates at a frequency lower than a maximum frequency of a plurality of operating frequencies, maybe one of a xcex9-shaped linear antenna and a V-shaped linear antenna, the xcex9-shaped linear antenna having antenna elements forming an angle less than 180 degrees at the feeder side, and the V-shaped linear antenna having antenna elements forming an angle greater than 180 degrees at the feeder side.
This offers an advantage of being able to adjust the radiation directivity at the operating frequency f1 by changing the shape of the linear antennas in accordance with an application purpose because the xcex9-shaped linear antennas will implement the radiation directivity of a wide beam at the front of the antenna at the operating frequency f1, whereas the V-shaped linear antennas will implement the radiation directivity of a narrow beam at the front of the antenna at the operating frequency f1.
Each of the antenna elements of the linear antennas with the cranks, which linear antennas operate at a frequency lower than a maximum frequency of a plurality of operating frequencies, may comprise linear conductors extending from connecting points of the cranks and a linear section of the antenna element to a direction opposite to a direction of the cranks.
This offers an advantage of being able to make impedance matching of the linear antennas with cranks operating at the frequency f1, when the multi-frequency array antenna operates at the frequency f1.
Each of the linear antennas that operate at a frequency lower than a maximum frequency of a plurality of operating frequencies may comprise an antenna element, a first half of a feeder and a crank, all of which are formed on a top surface of a dielectric board, and may comprise an antenna element, a second half of the feeder and a crank, all of which are formed on a bottom surface of the dielectric board.
This offers an advantage of being able to fabricate the linear antennas easily and accurately because the linear antennas are formed by printing them on the dielectric board by the etching process. In particular, the fabrication by the etching process is effective for an array antenna requiring a great number of antennas.
The multi-frequency array antenna may further comprise a crank length adjusting conductor provided to an upper portion of a protrusion constituting each crank formed on the antenna element.
This offers an advantage of being able to make fine adjustment of the radiation directivity of the linear antennas operating at the relatively high frequency f2 because the fine adjustment of the reradiation caused by the excitation current is made by adjusting the current excited in the linear antennas with the cranks.
Each of the cranks may comprise protrusions that are formed symmetrically with respect to a linear section of the antenna element constituting each of the linear antennas.
This offers an advantage of being able to adjust the impedance characteristics of the linear antennas with the cranks at the relatively high frequency f2 because the increasing number of the crank projections.